Ask HN: Looking for a Backup Email Relay

We operate a self-hosted email server, just a few users and very low volume. Our server delivers most of our outbound email directly to the destination servers, but, for various reasons, email from us to a few servers sometimes gets rejected. Up to now, the workaround has been to also have a Google G Suite account for our domain, and use the Google server to relay our email to those finicky domains. With Google G Suite/Workspace no longer being "free forever", we are looking for an alternative relaying server. Volume is minimal, probable fewer than 10 relayed emails per day. Any suggestions appreciated.